A mass flow controller as a kind of the flow rate control device measures a flow rate of a fluid flowing in a flow channel by the use of a flow rate sensor as shown in the patent document 1, and controls an opening degree of the fluid control valve so that a measured flow rate value of the fluid becomes a set flow rate value as being a target value. For this kind of the mass flow controller, an external output part such as an external output terminal is arranged to display the flow rate value of the fluid flowing in the flow channel by the use of a liquid crystal display while the flow rate is controlled or to make it possible for the other device to use the flow rate value. The external output part ordinarily displays the measured flow rate value output from the above-mentioned flow rate sensor as it is or the measured flow rate value with a minute fluctuation component removed, such as a noise component on the liquid crystal display.
Meanwhile, the mass flow controller is also used not only for controlling a flow rate so as to maintain a constant flow rate in a flow channel but also for making the flow channel have an entirely closed state wherein no fluid flows in the flow channel by setting the setting flow rate at zero.
Since the control valve is completely closed in the entirely closed state so that no fluid flows, it is ordinarily considered that the external output flow rate value output from the external output part is also considered to be zero.
However, if a pressure fluctuation is generated in the fluid on the upstream side where the flow rate sensor is arranged after the control valve is closed, a problem might occur in that the flow rate sensor outputs the measured flow rate value as if a fluid flows through the control valve and accordingly the external output part also outputs false information that the fluid flows in the control valve although no fluid actually flows on the downstream side of the control valve.
As mentioned, the following concrete problems are generated if the external output part outputs a value other than approximately zero although the control valve is actually completely closed and no fluid passes the control valve and no fluid flows on the downstream side of the control valve in the flow channel.
From a standpoint of a customer who actually uses the mass flow controller, the customer misunderstands if a fluid actually flows because it is so displayed as if the fluid flows and passes the control valve in spite of the entirely closed state by the use of the mass flow controller, or the customer mistakes that the mass flow controller is out of order although there is no problem in actual use.
From a standpoint of a manufacturer who conducts maintenance of the mass flow controller, in an entirely closed state it is difficult for the manufacturer to judge whether or not just a certain value is displayed as the external output flow rate value although there is no problem for the mass flow controller itself, or whether or not a value results from a problem such as drift or trouble with the flow rate sensor. In other words, it becomes difficult to use the external output flow rate value as the criteria for judging what maintenance is to be conducted.